GeoCape Airborne Simulator (GCAS) archived data README April 2022 PI: Scott Janz - scott.janz@nasa.gov Alternate contact: Laura Judd - laura.m.judd@nasa.gov Description: This data file contains GCAS NO2 column densities in molecules/cm^2, derived using measured spectra obtained during the TRACER-AQ flights in August-September 2021 ------Revision history----------- Release 0 (R0): Initial release of NO2 the vertical column retrieval. Release 1 (R1): Fixes to R0 which include some errors in geolocation as well as a stratospheric offset correction based on a daily TROPOMI retrieved value (ranging from 0.9-3.4e14 molecules cm-2). --------------------------------- Method: Absolute nadir radiances are measured and aggregated to 30 cross-track positions of approximately 250 m with an average along track length of 560 m. Note: Sample areas are approximate given typical flight altitudes and ground speed. Pixel bounds Lat/Lon data provide actual sample area used. Each average sample spectrum is calibrated using a nadir reference spectrum which is an average under clear-sky conditions over a scene with minimum observed NO2 absorption. For TRACER-AQ, this reference is from Sept. 25th between 19.4325-19.4525 UTC with coincident TROPOMI measurements of 1.2E15 molecules/cm2 in troposphere and 2.8E15 molecules/cm2 in stratosphere The result is analyzed in specific spectral windows to retrieve the molecular absorption using Differential Optical Absorption Spectroscopy techniques [1]. The software package developed at the Belgian Institute for Space Aeronomy (BIRA-IASB) QDOAS [2] is used to fit cross-section data to log-normalized spectra. Cross-sections are those used in the TROPOMI NO2 operational product. Average minimum sensitivity for the slant column product is ~0.6E15 molecules/cm2 and can be reduced by co-adding to a coarser spatial resolution. AMFs for slant-to-vertical column conversion are derived by running VLIDORT to calculate scattering weights using an a priori modeled atmosphere and BRDF parameters for surface reflectance from MODIS MCD43A1 averaged over the domain from June 2021 for land. Over water, an isotropic parameter 0.03 is used(median iso kernel over water from MCD43A1) plus the Cox-Monk kernel in VLIDORT to represent water surface reflectivity. NO2 modeled profiles are taken from GEOS-CF analysis. https://portal.nccs.nasa.gov/datashare/gmao/geos-cf/ Vertical columns are only calculated when roll angle of the aircraft is < 10 degrees. From previous campaigns, uncertainty related to potential error in the MODIS BRDF product (10%) results in ~ 5% uncertainty in tropospheric AMF. Some spatial patterns in the NO2 product match spatial features (e.g. lower NO2 over a darker tree-filled park vs. high NO2 over the bright surrounding city). Be cautious when analyzing these features as they may be artifact from not being fully represented in the retrieval. AMFs are temperature corrected following the guidance from TROPOMI NO2 ATBD. http://www.tropomi.eu/document/atbd-nitrogen-dioxide The retrieval assumes there are no clouds in the region, therefore the cloud_glint_flag should be applied when using this data. This flag is calculated using count_rates where the likelihood of a cloud and sun glint impacts are expected for count rates > 125000. All geolocation and geometry parameters refer to the center of the pixel sample area and take into account aircraft yaw, pitch, and roll as well as view angle. -------Relevant References for this retrieval with GCAS------------- Judd, L. M., Al-Saadi, J. A., Szykman, J. J., Valin, L. C., Janz, S. J., Kowalewski, M. G., Eskes, H. J., Veefkind, J. P., Cede, A., Mueller, M., Gebetsberger, M., Swap, R., Pierce, R. B., Nowlan, C. R., Abad, G. G., Nehrir, A., and Williams, D.: Evaluating Sentinel-5P TROPOMI tropospheric NO2 column densities with airborne and Pandora spectrometers near New York City and Long Island Sound, 13, 6113–6140, https://doi.org/10.5194/amt-13-6113-2020, 2020. Kowalewski, M. G. and Janz, S. J.: Remote sensing capabilities of the GEO-CAPE airborne simulator, SPIE Optical Engineering + Applications, San Diego, California, United States, 92181I, https://doi.org/10.1117/12.2062058, 2014. Nowlan, C. R., Liu, X., Janz, S. J., Kowalewski, M. G., Chance, K., Follette-Cook, M. B., Fried, A., González Abad, G., Herman, J. R., Judd, L. M., Kwon, H.-A., Loughner, C. P., Pickering, K. E., Richter, D., Spinei, E., Walega, J., Weibring, P., and Weinheimer, A. J.: Nitrogen dioxide and formaldehyde measurements from the GEOstationary Coastal and Air Pollution Events (GEO-CAPE) Airborne Simulator over Houston, Texas, 1–36, https://doi.org/10.5194/amt-2018-156, 2018. -------Fitting window boundaries and Cross-sections used------------- Cross-section compilation provided by Dr. Kelly Chance, Smithsonian Astronomical Observatory NO2 [325nm-360nm]: Vandaele, A.C., C. Hermans, P.C. Simon, M. Carleer, R. Colin, S. Fally, M.F. Merienne, A. Jenouvrier, and B. Coquart, Measurements of the NO2 absorption cross-section from 42000 cm-1 to 10000 cm-1 (238-1000 nm) at 220 K and 294 K, J. Quant. Spectrosc. Radiat. Transfer., 59, 171-184, 1998. Interference cross-sections: Ring: Chance, K., and R.L. Kurucz, An improved high-resolution solar reference spectrum for Earth's atmosphere measurements in the ultraviolet, visible, and near infrared, JQSRT,111, 1289-1295, 2010. H2O: HITRAN08 O4: Thalman, R. and Volkamer, R.: Temperature dependent absorption cross-sections of O2-O2 collision pairs between 340 and 630 nm and at atmospherically relevant pressure., Phys. Chem. Chem. Phys., 15(37), 15371–81, doi:10.1039/c3cp50968k, 2013. CHOCHO: R. Volkamer, P. Spietz, J.P. Burrows, and U. Platt, "High-resolution absorption cross-sections of glyoxal in the UV-vis and IR spectral ranges", J. Photochem. Photobiol. A: Chem. 172, 35-46 (2005); DOI: 10.1016/j.jphotochem.2004.11.011